Plasma CVD of carbonaceous films on substrate having reduced metal on its surface

ABSTRACT

A plasma CVD method for forming a carbonaceous material containing diamond or microcrystalline grains therein is disclosed, which comprises placing a substrate in a reaction chamber said substrate provided with reduced Ni, Ge, or Mn on its surface; inputting a carbon compound gas into said reaction chamber; supplying an electric energy to said gas to convert said gas to a plasma; and forming said cabonaceous material on said substrate, wherein said reduced Ni, Ge, or Mn act as seeds on said surface of said substrate to promote formation of said carbonaceous material.

This application is a continuation of Ser. No. 07/585,278, filed Sep.19, 1990, now abandoned, which was a continuation of Ser. No.07/387,800, filed Aug. 1, 1989, now abandoned, which was a divisional ofapplication Ser. No. 07/216,333, filed Jul. 8, 1988, now U.S. Pat. No.4,871,581.

BACKGROUND OF THE INVENTION

This invention relates to a carbon deposition method, and moreparticularly, relates to a microwave enhanced CVD method for performingcarbon depostion such as diamond.

Recently, ECR (Electric Cyclotron Resonance) CVD has attracted theinterest of researchers as a new method of manufacturing thin films,particularly amorphous thin films. For example, Matsuo et al disclosesone type of such as ECR CVD apparatus in U.S. Pat. No. 4,401,054. Thisrecent technique utilizes microwave energy to energize a reactive gassuch that it develops into a plasma. A magnetic field functions to pinchthe plasma gas within the excitation space. Within this excitationspace, the reactive gas can absorb the energy of microwaves. A substrateto be coated is located distant from the excitaion space (resonatingspace) for preventing the same from being spattered. The energized gasis showered onto the substrate from the resonating space. In order toestablish electron cyclotron resonance, the pressure in a resonatingspaces is kept at 1×10⁻⁵ Torr at which pressure electrons can beconsidered as independent particles and resonate with the microwaveenergy in an electron cyclotron resonance on a certain surface on whichthe magnetic field strength meets the requirement for ECR. The excitedplasma is extracted from the resonating space, by means of a divergentmagnetic field, and is conducted to a deposition space which is locateddistant from the resonating space and in which there is disposed asubstrate to be coated.

In such a prior art method, it is very difficult to perform carbondeposition of a polycrystalline or single-crystalline structure, so thatcurrently available methods are substantially limited to processes formanufacturing amorphous films. Also, high energy chemical vapor reactioncan not be readily accomplished by such a prior art and therefore it hasnot been possible to form diamond films or other films having highmelting points, or uniform films on a surface having depressions andcaves can not be formed. Furthermore, it was impossible to coat thesurface of a super hard metal such as tungsten carbide with a carbonfilm. Because of this it is necessary to coat a super hard surface witha fine powder of diamond for use of abrasive which has a sufficienthardness and to make sturdy mechanical contact between the diamondpowder and the substrate surface.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a microwaveenhanced CVD method for depositing a carbon.

Accoding to one aspect of the invention, in addition to a carboncompound gas, nickel, manganese or germanium compound gas is introducedinto the reaction chamber. The added compound functions as a catalyst.The reactive gas is excited to be a high density plasma at a pressure of0.001-760 Torr, preferably 10-100 Torr by virtue of the catalyst. Then,the excited condition of the carbon compound gas is maintained for asubstantial time while making contact with reduced nickel, manganese orgermanium elements which are produced by decomposition of the catalystcompound, and therefore resulting in high speed deposition of diamondgrains or carbon films containing diamond grains. "Reduced" nickel issuch that there is few oxygen atoms therein. For example, it can beobtained in accordance with the following equations by exposing nickelpowder to high temperature hydrogen gas or hydrogen plasma.

    NiO+H.sub.2 →Ni+H.sub.2 O

    or

    NiO+2*H→Ni+H.sub.2 O

By disposing an object to be coated in the mixed cyclotron resonancespace or a space distant therefrom being in an excited condition, theproduct can be deposited on the object. To this end, the object islocated in a position where the strength of the electric field ofmicrowave energy takes its maximum value or in the vicinity to thatposition. A reactive gas is introduced to the resonance space which ismaintained at a relatively high pressure between 0.001 Torr and 760Torr, preferably between 10 Torr and 100 Torr. Then, a high densityplasma is established which density is 10² to 10⁵ times as high as thatconventionally established. In the light of such a high density, itbecomes possible for the first time to decompose carbon compounds and toperform carbon deposition containing carbon as its main component, e.g.diamond deposition or i-carbon (composite comprising diamond ormicrocrystalline grains) deposition. In the case of deposition on asurface having indentations, depressions and caves, diamond has atendency to be deposited preferentially on the corner.

A gaseous catalyst is introduced simultaneously or after theintroduction of the carbon compound gas. the catalyst functions as ahomogeneous catalyst which is effective to flying carbon compounds. Incase of reduced nickel, manganese or germanium in the form of cores orseeds on the film formation surface, it can be function as"non-homogeneous" catalyst so that the carbon compounds make contactwith the catalyst on the surface and the excited condition can bemaintained for a substantial time period.

The formation mechanism of carbon depositions is such that the carboncompounds make contact, under a plasma-excited atmosphere, with theseeds of the catalyst which have been formed on the surface to becoated, resulting in crystal(s) grown around those seeds. During thedevelopment of the crystalline carbon on the surface, relatively lowdensity portions, e.g. amorphous portions, are removed by etching withplasma hydrogen or plasma oxygen, preferentially leaving relatively highdensity portions, e.g. crystals.

By virtue of this process in which a low dilution of hydrogen can beavailable, diamond deposition can be performed at a speed 5-20 times asfaster as those available in prior arts. For this reason, the productioncost per gram is on the order 100 times as expensive as those ofhigh-pressure press methods, while the production cost of vapor phasemethods is on the order 1000 times as expensive as those ofhigh-pressure press methods. Furthermore, uniform and even films ofdiamond becomes available for the first time by this invention which ispromising new applications.

According to another aspect of the invention, a new CVD process isproposed which utilizes a mixed cyclotron resonance. In the improvedexciting process, sonic action of the reactive gas itself must be takeninto consideration as a non-negligible perturbation besides theinteraction between respective particles of the reactive gas and themagnetic field and microwave, and as a result charged particles of areactive gas can be energized in a relatively wide resonating space.Preferably, the pressure is maintained higher than 3 Torr. For the mixedresonance, the pressure in a reaction chamber is elevated 10² -10⁵ timesas high as that of the prior art. For example, the mixed resonance canbe established by increasing the pressure after ECR takes place at a lowpressure. Namely, first a plasma gas is placed in ECR condition at1×10⁻³ to 1×10⁻⁵ Torr by inputting microwaves under the existence ofmagnetic field. Then a reactive gas is inputted into the plasma gas sothat the pressure is elevated to 0.1 to 300 Torr and the resonance ischanged from ECR to MCR (Mixed Resonance).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section view showing a CVD apparatus in accordancewith the present invention.

FIGS. 2(A) and 2(B) are graphical diagrams showing a computor simulationof the profiles of the equipotential surfaces of magnetic field in across section.

FIG. 3(A) and 3(B) are graphical diagrams showing equipotential surfacesrespectively in terms of magnetic field and electric fields of microwaveenergy propagating in a resonating space.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an examplary microwave enhanced plasma CVD appratusinaccordance with the present invention is illustrated. As shown in thefigure, the apparatus comprises a reaction chamber in which a plasmagenerating space 1 and an auxiliary space 2 are defined which can bemaintained at an appropriate negative pressure, a microwave generator 4,electro-magnets 5 and 5' in the from of Helmholtz coils surrounding thespace 1, a power supply 25 for supplying an electric power to theelectro-magnets 5 and 5', an a water cooling system 18.

The plasma generating space 1 has a circular cross section, and wthinthe plasma generating space, there is provided a substrate holder 10',made ofa material which provokes minimun disturbance of magnetic fieldcreated by the magnets 5 and 5' in the chamber, e.g., made of stainlesssteel or quartz. A substrate 10 is mounted on the holder 10'. Thesubstrate holder 10' is irradiated and heated to 800°-1000° C. in theatmosphere of a high temperature plasma gas by means of an infraredradiation 24, which is emitted from an IR heater 20, reflected from anIR reflection parabolic mirror 21 and focused on the back surface of theholder 10' through a lens 22. Reference numeral 23 designates a powersupply for the IR heater 20. Provided for evacuating the reactionchamber is an evacuating system comprising a turbo molecular pump 17 anda rotary pump 14 which are connected with the reaction chamber throughpressure controlling valves 11, 13 and 15. The substrate temperature mayreach a sufficient level solely by virtue of the plasma gas generated inthe reaction chamber and, in that case, the heater can be dispensedwith. Further, depending on the condition of the plasma, the substratetemperature might become too high to enable a suitable reaction tooccur, in which case cooling means for the substrate has to be provided.

In use of the above described apparatus, a substrate 10 is mounted on asubstrate holder 10', and the reaction chamber is evacuated to 1×10⁻⁶Torr or a higher vacuum condition. Then, hydrogen gas isintroduced froma gas introducing system 6 at 300 SCCM, and microwave energy at a powerlevel of 1 kilo Watts and a frequency of 2.45 GHz is injected from themicrowave generator through a microwave introduction window 15 into theplasma generating space 1 which is subjected at the same time to amagnetic field of about 2K Gauss generated by the magnets 5and 5'. Themagnets are adapted to adjust the magnetic field strength. The hydrogenis excited into a high density plasma state in the space 1. The surfaceof the substrate 10 is cleaned by high energy electrons and hydrogenatoms. In addition to the introduction of hydrogen gas, a productive gascomprising a carbon compound such as C₂ H₂, C₂ H₄, C₂ H₆, CH₃ OH, C₂ H₅OH or CH₄ for example is inputted at 30 SCCM through a gas introductionsystem 7. In this process, the productive gas is diluted with hydrogenat a sufficiently low density, e.g. 1 to 5%. Further, Ni(CO)₄ at 1 SCCM(and GeH₄ at 2 SCCM in addition, if necessary) as a catalyst is inputtedto the reaction chamber from the introduction system, the proportion ofthe catalyst to the carbon compound gas being 0.1% to 10%. NiF, NiO,NiF(H₂ O)n (where n=1.3), Ni(CN)₂, Ni(C₅ H₅)₂, GeH₄, GeF₄, manganesecarbonyl, MnF₂ and the like are examples of catalysts in accordance withthe present invention. They can be used respectively alone orcombinations. The CVD reaction which occurs results from carbon atomsbeing excited to a high energy condition and heated to 800°-1000° C. byvirtue of the plasma gas and the heater 20 so that the substrate 10mounted on the substrate holder 10' is coated with carbon in the form ofa 0.1-100 microns thick film of i-carbon (insulated carbon consisting ofmicrocrystals) or diamond having a grain diameter of from 0.1 to 100microns. In accordance with experimental tests, it took only two hoursto deposit a carbon film having an average thickness of 5 microns. Thedeposition speed can be increased by applying a bias voltage to thesubstrate holder. The carbon product in accordance with the presentinvention is characterized in that at least 50% of carbon atoms havebeen connected by sp³ bonds.

For reference, a film formation process was performed in the same mannerasin the above but without using a catalyst. As a result, it took 15hours toform a carbon film having an average thickness of 4 microns. Itwas confirmed by a metal microscope (1000 times magnification) that theunevenness of the surface of the film was significant. In accordancewith the present invention, since innumerable seeds of catalyst prevailover the surface to be coated, carbon films can be formed with flatsurfaces.

Next, a second embodiment of the present invention will be described. Inthis embodiment, a gaseous catalyst and hydrogen are introduced to thereaction chamber in advance of the introduction of a carbon compound gasin order to form innumerable clusters of reduced nickel or germaniumwhichfunction as seeds on a surface to be coated. After that, a carboncompound gas and hydrogen are introduced to initiate carbon deposition.In accordance with experiments, the carbon films could be formed withsmooth surfaces. In this process, however, since the catalyst does notmake contact with the gaseous carbon compound being excited by plasmagas, the deposition speed is improved only by 20% to 30% in comparisonwith those available in prior arts. The reason why the improvement ofthe deposition speed is so limited as compared with the previousembodiment is that the catalyst is covered with carbon and preventedfrom further contacting the carbon compound.

FIG. 2(A) is a graphical showing of the distribution of the magneticfield in the region 30 in FIG. 1. The curves in the diagram are plottedalong equipotential surface and are given numerals indicating thestrengths along the respective curves of the magnetic field induced bymagnets 5 and5' having a power of 2000 Gauss. By adjusting the power ofthe magnets 5 and 5', the strength of the magnetic field can becontrolled so that the magnetic field becomes largely uniform over thesurface to be coated whichis located in the region 100 where themagnetic field (875±185 Gauss) and the electric field interact. In thediagram, the reference 26 designates the equipotential surface of 875Gauss at which the conditions required for ECR (electron cyclotronresonance) between the magnetic fieldand the microwave frequency aresatisfied. Of course, in accordance with the present invention, ECR cannot be established due to the high pressurein the reaction chamber, butinstead a mixed cyclotron resonance takes place in a broad regionincluding the equipotential surface which satisfied ECR conditions. FIG.2(B) is a graphical diagram in which the X-axis corresponds to that ofFIG. 2(A) and which shows the strength of the electric field of themicrowave energy in the plasma generating space 1. As shown, theelectric field strength takes its maximum value in the regions 100 and100', it is difficult to heat the substrate 10' without disturbing thepropagation of the microwave energy. In other regions, a film will notbe uniformly deposited, but will be deposited in the form ofa doughnut.It is for this reason that the substrate 10 is disposed in the region100. The plasma flows in the lateral direction. According toexperiments, a uniform film can be formed on a circular substrate havingadiameter of up to 100 mm, and a film can be formed in the chamber on acircular substrate having a diameter of up to 50 mm with a uniformthickness and a uniform quality. When a larger substrate is desired tobe coated, the diameter of the space is doubled with respect to thevertical direction of FIG. 2(A) by making use of 1.225 GHz as thefrequency of the microwave energy. FIG. 3(A) and FIG. 3(B) are graphicaldiagram showing the distribution of the magnetic field and the electricfield due to microwave energy emitted from the microwave generator 4 fora cross section of the plasma generating space 1. The curves in thecircles of thefigures are plotted along equipotential surfaces and givennumerals showingthe field strengths. As shown in FIG. 3(B), the electricfield reaches its maximum value at 25 KV/m.

In accordance with the present invention, carbon films or clusters canbe formed. The effect of the invention has been confirmed in regard tocarbondeposition, and therefore it is advantageous to apply the presentinventionto the formation of any films containing carbon whoseproportion is not lower than 50%.

While a description has been made for several embodiments, the presentinvention should be limited only by the appended claims and should notbe limited by the particular examples, and there may be caused toartisan some modifications and variation according to the invention. Forexample, it has been proved effective to add boron, nitrogen, phosphorusor the like into the carbon. This invention is applicable to other typesof CVD methods such that a plurality of substrates have been arranged inparallelwith each other and parallel with the propagating direction ofthe microwave, and in this case a 13.56 MHz electric power is suppliedto the reaction chamber in which there are disposed the substrates givena bias voltage of 50 KHz.

I claim:
 1. A method for forming a carbonaceous material containingdiamond or microcrystalline grains therein comprising the stepsof:placing a substrate in a reaction chamber said substrate providedwith reduced Ni, Ge, or Mn on its surface; inputting a carbon compoundgas into said reaction chamber; supplying an electric energy to said gasto convert said gas to a plasma; and forming said carbonaceous materialon said substrate wherein said reduced Ni, Ge, or Mn act as seeds on thesurface of said substrate to promote formation of said carbonaceousmaterial.
 2. A method as in claim 1, wherein said carbon compound gascomprises a hydrocarbon.
 3. A method as in claim 1, wherein saidelectric energy is a microwave energy.
 4. A method for forming acarbonaceous material containing diamond or microcrystalline grainstherein comprising the steps of:placing a substrate in a reactionchamber; inputting a catalyst gas comprising a compound of Ni, Ge, or Mnwith a hydrogen gas into said reaction chamber; introducing an electricenergy into said reaction chamber to excite said catalyst gas; forming areduced Ni, Ge, or Mn from said catalyst gas on said substrate;inputting a carbon containing gas into said reaction chamber in orderthat said carbon containing gas is excited; and then forming saidcarbonaceous material on said substrate wherein said Ni, Ge, or Mn actas seeds on the surface of said substrate to promote formation of saidcarbonaceous material.
 5. A method as in claim 4, wherein said carboncompound gas comprises a hydrocarbon.
 6. A method as in claim 4, whereinsaid electric energy is a microwave energy.
 7. A method as in claim 4,wherein said catalyst gas comprises a compound selected from the groupconsisting of nickel carbonyl, GeH₄, NiF(H₂)n Ni(CN)₂, Ni(C₅ H₅)₂, GeH₄,manganese carbonyl, and MnF₂.